Adhesives derived from protein-containing soy flour first came into general use during the 1920's (see, e.g., U.S. Pat. Nos. 1,813,387, 1,724,695 and 1,994,050). Soy flour suitable for use in adhesives was, and still is, obtained by removing some or most of the oil from the soybean, yielding a residual soy meal that was subsequently ground into extremely fine soy flour. Typically, hexane is used to extract the majority of the non-polar oils from the crushed soybeans, although extrusion/extraction methods are also suitable means of oil removal.
The resulting soy flour was then, generally, denatured (i.e., the secondary, tertiary and/or quaternary structures of the proteins were altered to expose additional polar functional groups capable of bonding) with an alkaline agent and, to some extent, hydrolyzed (i.e., the covalent bonds were broken) to yield adhesives for wood bonding under dry conditions. However, these early soybean adhesives exhibited poor water resistance, strictly limiting their use to interior applications.
In addition, soybean adhesives common in the prior art exhibit a limited pot life. After only a few hours, the viscosity and performance of the alkaline-denatured soy flour mixture rapidly decreases. This reduction in performance is believed to be a result of some hydrolysis of the soy flour and the excessive breakdown of the secondary, tertiary and quaternary structures deemed to be important for the formation of both strong adhesive and cohesive bonds. Thus, a need exists for an adhesive demonstrating a balance between exposing sufficient functional groups for improved performance while retaining enough protein structure to maintain adhesive performance and offering stability.
In the 1920's, phenol-formaldehyde (PF) and urea-formaldehyde (UF) adhesive resins were first developed. Phenol-formaldehyde and modified urea-formaldehyde resins were exterior-durable, but had high raw materials costs that initially limited their use. World War II contributed to the rapid development of these adhesives for water and weather resistant applications, including exterior applications. However, protein-based adhesives, mainly soy-based adhesives that were often combined with blood or other proteins, continued to be used in many interior applications.
Currently, interior plywood, medium-density fiberboard (MDF) and particleboard (PB) are primarily produced using urea-formaldehyde resins. Although very strong, fast curing, and reasonably easy to use, these resins lack hydrolytic stability along the polymer backbone. This causes significant amounts of free formaldehyde to be released from the finished products (and ultimately, inhaled by the occupants within the home). There have been several legislative actions to push for the reduction of formaldehyde emissions when used in interior home applications (Health and Safety Code Title 17 California Code of Regulations Sec. 93120-93120.12, and the new United States national standard—Reference: 2010 U.S. S1660).
Soy-based adhesives can use soy flour, soy protein concentrates (SPC), or soy protein isolates (SPI) as the starting material. For simplicity, the present disclosure refers to all soy products that contain greater than 20% carbohydrates as “soy flour”. Soy flour is less expensive than SPI, but soy flour contains significant levels of activated urease (an enzyme that rapidly and efficiently decomposes urea into ammonia), thus requiring a need for the urease to be deactivated when using urea in the final adhesive. This needs to be accomplished without compromising the viscosity/solids ratio or performance of the final product. Soy flour also contains high levels of carbohydrates, requiring more complex crosslinking technique, as crosslinking results in the much improved water resistance of the soy-based adhesives.
SPC contains a greater amount of protein than soy flour, but contains less protein than SPI. Typically, SPC is produced using an alcohol wash to remove the soluble carbohydrates.
SPI is typically produced through an isoelectric precipitation process. This process not only removes the soluble sugars but also removes the more soluble low molecular weight-proteins, leaving behind mainly high molecular weight-proteins that are optimal for adhesion even without modification. As a result, SPI makes a very strong adhesive with appreciable durability. However, SPI is quite costly, and is therefore not an ideal source of soy for soy-based adhesives. Thus, there is a strong need to produce high quality adhesives from soy flour.
U.S. Pat. No. 7,252,735 to Li et al. (Li) describes soy protein crosslinked with a polyamido-amine epichlorohydrin-derived resin (PAE). Li describes these particular PAEs, which are known wet strength additives for paper, in many possible reactions with protein functional groups. In Li, SPI is denatured with alkali at warm temperatures and then combined with a suitable PAE resin to yield a water-resistant bond. This aqueous soy solution must be prepared just prior to copolymerization (or freeze-dried) to allow for a suitable pot life. Li does not teach or suggest the importance of denaturing soy for use with PAE, as the SPI used in Li already has an extensive thermal history. Moreover, the alkali process described by Li is not sufficient to deactivate the urease in soy flour and is, therefore, not a suitable approach to make soy flour/urea adhesives. Furthermore, the adhesives described by Li suffer from at least one of the following: high viscosity, low solids, or poor stability.
U.S. Pat. No. 6,497,760 to Sun et al. (Sun) also teaches soy-based adhesives made from SPI as a starting material. Sun teaches that the SPI can be modified with urea, but Sun does not teach or suggest modifying soy flour with urea to provide an improved soy flour-based adhesive. Urea is a known denaturant for adhesives having no significant urease activity, such as SPI. However, urea is problematic for soy flours as they contain moderate to high levels of urease activity. While it is known that SPI can be denatured with urea (see, e.g., Kinsella, J. Am. Oil Chem. Soc., March 1979, 56:244), Sun teaches away from using urea with soy flour because of the urease activity associated with it.
There is very limited previous work the describes any method(s) to deactivate the urease in soy flour and there is no such work that describes this particular acid treatment approach.
U.S. Pat. No. 3,220,851 to Rambaud describes a method of treating soya beans to improve their quality and usability in food processing. Rambaud describes cooking the soya in an aqueous solution to temperatures not to exceed 80° C. so as to remove the “undesirable” compounds such as urease and antitrypsin from the soya. Rambaud specifically teaches that the temperature of 80° C. constitutes a threshold value beyond which the speed of the degradation of the albumins increases rapidly, and it is therefore essential not to exceed this value. Rambaud also does not teach or suggest why removing urease or antitrypsin may be useful for the soya beans with respect to their ability to serve as adhesives.
Wescott (U.S. application Ser. No. 11/779,558) also teaches a higher temperature method for treating soy to deactivate the urease. This method, although effective in deactivating the urease, is inferior to this invention in that results in a significant increase in viscosity and color as compared to this invention.
U.S. Pat. No. 7,345,136 to Wescott describes a method for denaturing soy flour in preparation for copolymerization by the direct addition of formaldehyde. Such a method, if applied to this invention would result in high ammonia levels and significant performance decreases. Alternatively, if the method of this invention is applied to the process of Wescott (U.S. Pat. No. 7,345,136) immediate gelation is realized when formaldehyde is added to the denatured soy flour. This is a result of an insufficient level of denaturation for the process.